Currently, a solid oxide fuel cell (hereinafter referred to as “an SOFC”), currently referred to as a third-generation fuel cell, adopts thermochemically stable zirconia as an electrolyte with a fuel electrode serving as an anode and an air electrode serving as a cathode attached thereto. In the SOFC, a fuel gas such as hydrogen, methane, methanol, diesel or the like may be used without reformation, and an oxidizing agent such as air or oxygen is employed. Thus, SOFCs are receiving attention as high-efficiency low-pollution electric power generation technology. The SOFC utilizes as an electrolyte yttria-stabilized zirconia having a stable crystalline structure. This material exhibits oxygen ion conductivity which is characteristically governed by the temperature, and the desired conductivity for the fuel cell is attainable at 800˜1000° C. Therefore, the SOFC is typically operable at a temperature of 800˜1000° C. and thus adopts ceramics for the electrode material as they can withstand such a high temperature. For example, the material for the cathode to which air is introduced includes LaSrMnO3, and a material for the anode at which hydrogen is introduced includes a Ni—ZrO2 mixture.
In a conventional planar SOFC, a unit cell is formed by respectively coating front and back sides of an electrolyte plate serving as a support with an air electrode material and a fuel electrode material, performing a sintering process, thus forming electrolyte-electrode assemblies having a predetermined thickness, and then disposing an interconnector made of a conductive metal material between the electrolyte-electrode assemblies so that the interconnector electrically connects cathodes and anodes of upper and lower unit cells which are to be stacked. Such an SOFC also has gas channels for supplying fuel and air in both sides thereof. Such a planar fuel cell is advantageous because the electrolyte-electrode assembly is thin, but uniformity or flatness of the thickness is difficult to adjust due to the properties of the ceramic, thus making it difficult to increase the size of the fuel cell. Further, when the electrolyte-electrode assemblies and the interconnectors are alternately layered in the unit cell stack, all of the edge portions of the unit cells should be provided with a gas sealing material in order to prevent the gases of the upper and lower unit cells from mixing. Although glass which is useful as the sealing material begins to soften starting at about 600° C., it is preferred in terms of efficiency that the SOFC be typically operated at a high temperature of about 800° C. or higher. However, a perfect sealing material has not yet been found. In addition, in the unit cells for a fuel cell, there is the dangerous probability of thermal and mechanical stress during heating or cooling causing structural instability, and also, there is a high risk of gas leakage because of the crystallization of the sealing material. This makes it difficult to increase the size of the unit cells. Therefore, the planar cell is required to be further improved in various aspects in order for it to be commercialized.
With the goal of overcoming the problems of the planar cell, a cylindrical cell is disclosed in U.S. Pat. Nos. 6,207,311 B1 and 6,248,468 B1. Compared to the planar cell, the cylindrical cell has slightly lower stack power density but is significantly advantageous in terms of strength and gas sealing. Accordingly, a unit fuel cell using the cylindrical cell is formed by sequentially disposing an air electrode, an electrolyte, a fuel electrode and an interconnection layer on a porous support tube made of zirconium oxide or the like. The cylindrical cell is advantageous in that there is no need for a gas sealing material in the cell, and thus ceramic sealing problems as in the planar cell do not occur. Further, each cell is formed on a solid support, the fuel cell itself constitutes a strong ceramic structure, and the resistance to thermal expansion is high. Furthermore, because contact between the cells occurs in a reducible atmosphere, an interconnector made of a metal material may be used. However, in the case where a plurality of unit cells is connected to each other to form a stack in order to increase the capacity of the fuel cell, power current flows along a thin electrode surface in a longitudinal direction, undesirably increasing internal resistance, making it impossible to increase the size of the fuel cell. To draw out current in a radial direction in order to overcome the above problems, the inside or outside of each tube should be provided with an interconnector or wound with a wire. Also, because tubes should be disposed at predetermined intervals so as not to make contact with each other upon connection of the plurality of unit cells, unnecessary space is increased, resulting in the loss of the high power density per unit volume.
Recently, in order to solve the problems of SOFCs which use the planar cell and the cylindrical cell, there have been developed a unit cell and a unit cell stack using a flat tube type structure for increasing power density which also solves the sealing problems of the planar cell by manufacturing a fuel cell module having both a planar cell structure and a cylindrical cell structure, as disclosed in Korean Patent Laid-Open Publication No. 10-2005-0021027 and U.S. Pat. Nos. 6,416,897 and 6,429,051. Even in this case, however, gas flow passages for supplying air or fuel electrode gases and an interconnector should be essentially provided to the outside of the flat tube type cell. Such an interconnector increases the mechanical strength of the stack and enlarges the contact area of unit cells, thus increasing power density, but because the interconnector is made of a metal, mechanical and thermal stress undesirably occurs between the electrolyte-electrode assemblies made of ceramic upon high-temperature operation. During long-term use at high temperatures, corrosion may occur due to air on the surface of the interconnector, and when the size of the flat tube type cell is increased, it is not easy to solve the thermomechanical stress between the ceramic material and the metal material.
As described above, because the solid oxide fuel cell is manufactured using a ceramic material, it is difficult to increase the unit cell area. Moreover, if the unit cells are physically or electrically connected with each other in series alone, when the performance of a specific cell is deteriorated, the entire performance of the stack is deteriorated. Due to these problems, all the cells should be perfectly manufactured and operated, but this is a task too difficult to achieve. Furthermore, when a specific cell in the stack breaks down or the performance thereof is reduced, the cell is difficult or impossible to replace or repair. In general, solid oxide fuel cells can be operated in a highly efficient manner at significantly high temperatures compared to other fuel cells, including polymer electrolyte fuel cells and molten carbonate fuel cells, and can oxidize even CO and the like. These may use various types of fuels, including coal gas, biogas and diesel gas, and can also be used in power plants having a large capacity of 1 MW or higher, and high-temperature off gas from the solid oxide fuel cells enables the generation of an additional amount of electricity. Thanks to these advantages, solid oxide fuel cells are the most promising and commercially competitive. However, due to the above-described multitude of problems, it is actually impossible to make a huge unit cell area and increase the stack capacity to 1 MW or higher.
In fuel cell reactions, a large amount of heat is generated by the oxidation of hydrogen. Thus, when the area of unit cells is increased or the number of stacks is increased to manufacture a large-sized stack, it is impossible to control the variation in temperature between the central portion and the peripheral portion of the stack, and this phenomenon causes more serious problems in solid oxide fuel cells which are operated at high temperatures. Meanwhile fuel cells use hydrogen as the reaction gas. Generally, hydrogen is produced by reforming a hydrocarbon-containing fuel gas with steam, and this hydrogen production reaction is endothermic. Thus, if a reformer is interposed between unit cells or if the anode layer or interconnection plate of a unit cell is coated with a reforming catalyst so as to cause reforming reactions at the same time, a problem in heat generation in fuel cell reactions can be controlled. However, unfortunately, an anode layer of Ni-zirconia cermet which is currently used has an excellent activity for reforming hydrocarbons, but ultimately breaks down due to severe coking caused by Ni at high temperature, and thus cannot be exposed directly to a hydrocarbon-containing fuel gas. Due to this problem, it is impossible to control the heat caused by a fuel cell reaction, if a separate reformer is not physically inserted between a unit cell and an interconnection plate.